1. Field of the Invention
The present invention relates to image pickup apparatuses and methods for controlling the same, and more particularly, to an image pickup apparatus for applying dark shading correction and a method for controlling the same.
2. Description of the Related Art
For image pickup elements such as charge-coupled device (CCD) sensors and complimentary mental-oxide semiconductor (CMOS) sensors used in image pickup apparatuses such as digital cameras, various dark shading correction methods for correcting image data for dark noise have been proposed in the related art.
FIG. 10 is a schematic block diagram of an exemplary functional structure of a digital camera for applying existing dark shading correction. The camera shown in FIG. 10 takes a photograph of an object using an image pickup element 1001 under image pickup conditions (aperture, shutter speed, and sensitivity) determined by an exposure control operation, and reads electric charge signals accumulated in the image pickup element 1001. The read electric charge signals are converted into digital signals by an analog-to-digital (A/D) converter 1002, and the digital signals are stored as image signals (hereinafter referred to as “object image signals”) in a buffer memory 1003. Next, under the same image pickup conditions, the camera takes a photograph with the image pickup element 1001 shielded from light to obtain electric charge signals, and the electric charge signals are converted into digital signals by the A/D converter 1002. The digital signals or image signals obtained in this manner are referred to as “black image signals”.
At the same time as the black image signals are output from the A/D converter 1002, the object image signals stored in the buffer memory 1003 are read. A subtracting circuit 1004 subtracts the black image signals from the corresponding object image signals, thereby applying dark shading correction to the object image signals. An arbitrary offset value (e.g., about 32 least significant bits (LSB) of a 10-bit signal) is added to each of the dark-shading-corrected object image signals by an offset circuit 1005, and the sum signals are sent to a signal processing circuit 1006, which in turn generates image signals representing one image. The image signals generated in this manner are stored in a general storage medium 1007, such as an SD card.
However, since the above-described dark shading correction method uses image signals representing two images, namely, the object image signals and the black image signals, random noise is increased √2 times, compared with the case where no dark shading correction is applied. Since two images are captured and read, it takes time to complete the image pickup operation.
For example, Japanese Patent Laid-Open No. 11-289494 describes a method configured to divide black image signals into multiple blocks, integrate signals on a block-by-block basis, enlarge the integrated signals to a resolution of an object image, and subtract the enlarged signals from object image signals. According to this method, random noise of the black image signals is reduced. Therefore, deterioration of the dark-shading-corrected image signals due to noise can be reduced.
However, even in the method described in Japanese Patent Laid-Open No. 11-289494, in order to obtain image signals representing one object image, as has been described with reference to FIG. 10, image signal capturing and reading operations are performed twice. Therefore, it takes time to complete the image pickup operation. In a continuous shooting mode, the speed of taking photographs in a continuous manner drops greatly.